1. Field of the Invention
The present invention relates to an image reading apparatus for reading image information of an original and is applied to an image forming apparatus, for example, such as an facsimile machine and the like.
2. Related Background Art
Heretofore in the past, this kind of the image reading apparatus has been available such as shown in FIG. 15. As shown in FIG. 15, an original pressing means 52 for pressing the original to a contact image sensor 51 as an image reading means was configured in such manner that it is contacted to the reading surface of the contact image sensor 51.
The original pressing means 52 is configured by a white sheet 52a as a reading reference (a color reference) and a white sheet metal 52b. The white sheet 52a is fixed with its upper portion screwed with a vis. The white sheet metal 52b is fixed by hanging on an upper original guide above the inside of the apparatus at an original conveying direction downstream side.
Also, as shown enlarged in FIG. 16, an emboss 52c is disposed outside the original conveying direction area of a longitudinal both ends portion of the original pressing means 52 so that the original pressing means 52 is not adhered (or cohered) to the reading surface of the contact image sensor 51.
Note that the original pressing means 52 at the time when the above described emboss 52c is disposed is deprived of a function to press the original to the reading surface and given a function to guide the original at a reading location so as not to behave violently.
And, for example, at the time of the pre-scanning prior to the commencement of the image reading operation, the operation for detecting a reflective light volume from the reading reference 52a and for deciding a white level (a color reference) to determine the output level (the contrast level) of the information read by the image sensor is activated.
However, in case of the above described prior art, the white sheet 52a strongly contacts the reading surface of the reading location of the contact image sensor 51 and tends to adhere (or cohere) thereto owing to the fixing condition of the original pressing means 52 to the contact image sensor 51, accuracy of each component thereof and the like. Then, the reflective state of light at the adhesive (or cohesive) surface is changed and the output of brightness which is the reading reference of the image is lowered, thereby causing the problem where the reading image becomes whitish. When this adhesion (or cohesion) arises at a longitudinal part, a malfunction occurs in which the portion where the output is lowered becomes white line (or streak) if seen from the whole reading image.
For this reason, in order to prevent the adhesion (or cohesion) of the original pressing means 52, it was necessary to take measures such as the disposition of an emboss of high accuracy and the control of warping of the white metal sheet and the like.